The main demands on human skin are to prevent loss of water, and to prevent water and other matter of the environment from entering the body indiscriminately. The human skin thus forms a selectively permissible physical barrier between the human body and its surroundings.
The barrier function has been shown to reside in the stratum corneum. The stratum corneum is the topmost layer of the skin, and is built of corneocytes. Corneocytes are cells that contain extensively cross-linked proteins, surrounded by a highly resistant cell envelope. The corneocytes are embedded in a bed of specific lipid structures of long chain lipids. These long chain lipids are organized as bilamellar structures stacked on top of each other. The bilamellar structures fill the intercellular spaces between the corneocytes.
To account for the skin's barrier properties, and for its selective hydrophilic and hydrophobic pathways, the skin has been described as a mosaic barrier model. This model envisages barrier lipids to exist predominantly in crystalline (gel) form. Such a form provides water impermeable domains, which are surrounded by so-called grain borders of lipids in a liquid crystalline state. This arrangement provides an effective, water tight barrier that still allows a minute but controlled loss of water through the liquid crystalline interdomains. This controlled water loss is enough to keep the keratin of the stratum corneum hydrated. The liquid character of the interdomain grain borders allows passage of hydrophilic and hydrophobic molecules on down-hill gradients, i.e. passage by passive diffusion.
Dermal delivery systems are compositions which deliver active substances to, or through, the skin. These compositions typically contain skin permeation enhancers. Permeation enhancers may induce structural transformations of the bilamellar structure in the liquid crystalline interdomain regions, and thus promote transdermal delivery of, for example, pharmacological substances.
Typical dermal delivery systems have an alcohol or petroleum base, with little consideration given to the biological properties of the vehicle itself. For example, emulsified fatty acids can inherit certain detergent properties if their structure is significantly altered from those in the normal skin. The detergent properties can lead to disruption of the normal barrier function, which is counteractive to the potential benefit of the delivery system. Disruption of the normal barrier function often causes the stratum corneum to lose its natural potential to function properly as a barrier. As a result, the skin becomes either too dry or too permeable to environmental substances.
Other conventional delivery systems that are thought to protect the skin from harmful substances are barrier ointments. The purpose of barrier ointments is to provide a film, and thereby create a layer which is impermeable to environmental substances. Due to the impermeability, though, these ointments both increase the body temperature of the treated body part, as well as prevent perspiration, and thus render an uncomfortable sensation.
The dermal delivery systems described above are not formulated to deliver a substance to, or through, the human skin without permanently disrupting the stratum corneum's natural barrier function.